Principal Species - Online Tutor, Practice Problems & Exam Prep

The acid dissociation constant (Ka) indicates acid strength; a Ka > 1 signifies a strong acid, while Ka < 1 indicates a weak acid. The relationship between pH and pKa helps determine the predominant species in a solution. For a monoprotic acid, if pH < pKa, the acidic form predominates; if pH > pKa, the basic form is favored. When pH equals pKa, both forms are present in equal amounts. This concept extends to diprotic and polyprotic acids, enhancing understanding of acid-base reactions and equilibria.

Principal Species

We've learned in the past that our acid dissociation constant, otherwise known as Ka, is a way of gauging the strength of an acid. We know that if our Ka value is greater than 1, then we're dealing with a strong acid. If our Ka is less than 1, then we're dealing with a weak acid. In addition to this, the acid constant Ka tells us the numerical value that indicates how easily an acidic hydrogen can be removed. Basically, if we convert our Ka to pKa, we can compare it to the pH of our solution to determine which one is the principal species within our solution.

Is it the acidic form or the basic form that predominates? Here, if we're looking at a monoprotic acid and the pH is less than our pKa, then we would say that the acid form is greater than the basic form. On the other hand, if the pH is greater than our pKa, then the acid form is less than the basic form. You can think of it as having these two forms, H_A and A^-, with a dividing line between them, representing our pKa. If your pH happens to be less than this pKa, then you exist in your acidic form. However, if you crank up the pH so that it surpasses the pKa, you exist in your basic form.

Now, what would be the principal species if your pH equaled your pKa? If your pH equaled your pKa, you would be exactly on this dividing line. On this line, both forms exist together. If your pH equaled your pKa, that would mean that you have 50% of the acid form and 50% of the basic form. This relationship of pH and pKa can also be expanded to diprotic as well as polyprotic acids.

We'll take a look at those situations as we investigate each of the example problems left below. So, click on the next video and see how we approach example 1.

Principal Species

So in this question here, it says, What is the predominant form of the diprotic acid, methionine at a pH equal to 4.18? All right, so we're dealing with a diprotic acid. Remember, when it comes to diprotic species, there are 3 major forms. We have our fully protonated form. We can say that is H₂A.

You could think of this as the acidic form. Then we have our intermediate form, HA^-. And finally, we have our basic form after all the acidic hydrogens have been removed. That is A^2-. Remember, when we're talking about boundary lines and things, so in between our acidic form and the intermediate form, there exists one boundary here.

We're talking about removing the first acidic hydrogen from H₂A to create HA^-. Since we're talking about the first acidic hydrogen, that means we're dealing with K₁. Here, taking the negative log of that gives me 2.18. Here on this border, pK_a1 equals 2.18. Remember, exactly on that border, we'd have 50% of the acid form and 50% of the intermediate form in this case.

The next border that separates my intermediate form from the basic form would be here. In that case, we'd be dealing with pK_a2 because we're removing the second acidic hydrogen to create our basic form. PK_a2 equals negative log of K_a2. That's negative log of 8.3 times 10^-10. That gives me 9.08.

Here we'd have 50% of the intermediate form and 50% of the basic form. Now here, we're dealing with the pH equal to 4.18 which means that we've passed beyond this border here. We're not quite at this border here. We're somewhere in the middle. What form exists predominantly in that space?

The intermediate form does. Here, the intermediate form will be the predominant form because it is past the first border. It's blown past the acidic form but hasn't quite reached the second border. It's not having, any equal equal amount with the basic form. In this case, our intermediate form of methionine would be the predominant species.

Now that you've seen this example, see if you can do example 2. Once you do, come back and see how I approach that same question.

Principal Species

Although we didn't cover one dealing with a triprotic acid, we can apply the same methods that we used in the previous example. Here it says, "What is the predominant form of histidine at a pH equal to 8?" Here we know it's a triprotic acid because it has 3 pKa values.

Principal Species

If we take a look at this example 3, we said, what is the 2nd most predominant form in the previous question? We know that the predominant form is the intermediate 2 form. This pH of 8 which is here is within this region. Next to that region is the 1st intermediate form and our basic form. We're trying to determine which one of these 2 would be the next highest concentration at a pH of 8.0.

Now, I gave the hint of looking up the isoelectric point. Here, if we calculate the isoelectric pH, that'll help us navigate which one will be the second most dominant form. Here, the equation for pH is pH=12(pKa1+pKa2). Which pKa's are we dealing with? pKa2 and pKa3.

Those would be the pKa's that we're using. That equals half of 5.97 plus 9.28. We're going to figure out the pH here. When we do, we get 7.625 as my pH, my isoelectric pH. Now, what do we say in terms of this value?

Well, we're going to say here, if we are above a pH of 7.625, that means we are more basic. Right? Because the higher the pH gets, the more basic your solution becomes. And the more basic your solution becomes, the easier it is to deprotonate or take off an H+. So at a pH above 7.625, it becomes too basic.

So the second dominant form will be A3- basic form would be the 2nd most predominant. And then if we said below a pH of 7.625, that would mean that our pH is lower so it's more acidic. Remember, in a more acidic environment, what do we do? We add an H+ to something.

We protonate it. In that form, H2A- which is the intermediate one would be the 2nd most predominant. This whole isoelectric pH formula is just to help us establish a baseline. All we do now is we compare, this new baseline to the current pH environment. The current pH environment is 8.0 like we said in example 2.

The current pH value is higher than our baseline of 7.625. Therefore, it's the basic form that would be the 2nd most predominant form within the solution. That's how we are able to determine who's the 2nd most popular or dominant form within a solution. Refer to the borders, examples for examples 1 and 2 to find the most dominant, and then refer to an isoelectric pH to determine which one is second. Once you do that, you'll be able to get your answers.

Here’s what students ask on this topic:

What is the relationship between pH and pKa in determining the principal species in a solution?

The relationship between pH and pKa is crucial in determining the principal species in a solution. If the pH is less than the pKa, the acidic form (HA) predominates. Conversely, if the pH is greater than the pKa, the basic form (A^-) predominates. When the pH equals the pKa, both the acidic and basic forms are present in equal amounts, meaning the solution contains 50% of each form. This relationship helps in understanding the behavior of acids and bases in different pH environments and is applicable to monoprotic, diprotic, and polyprotic acids.

How does the acid dissociation constant (Ka) indicate the strength of an acid?

The acid dissociation constant (Ka) is a measure of the strength of an acid in solution. A Ka value greater than 1 indicates a strong acid, which dissociates completely in water. Conversely, a Ka value less than 1 indicates a weak acid, which only partially dissociates. The Ka value provides insight into the extent of dissociation and the ability of the acid to donate protons (H⁺). This information is essential for predicting the behavior of acids in various chemical reactions and environments.

What happens to the principal species when pH equals pKa?

When the pH of a solution equals the pKa of the acid, the principal species are present in equal amounts. This means that 50% of the species is in the acidic form (HA) and 50% is in the basic form (A^-). This equilibrium point is significant because it represents the pH at which the acid and its conjugate base coexist in equal concentrations, providing a balanced state between the two forms.

Can the concept of pH and pKa be applied to diprotic and polyprotic acids?

Yes, the concept of pH and pKa can be extended to diprotic and polyprotic acids. These acids have multiple dissociation steps, each with its own pKa value. For each dissociation step, the same principles apply: if the pH is less than the pKa, the acidic form predominates; if the pH is greater than the pKa, the basic form predominates. When the pH equals a specific pKa, the corresponding acidic and basic forms are present in equal amounts. Understanding these multiple equilibria is essential for analyzing the behavior of complex acids in solution.

How can you use pKa to predict the predominant form of a monoprotic acid in a solution?

To predict the predominant form of a monoprotic acid in a solution, compare the pH of the solution to the pKa of the acid. If the pH is less than the pKa, the acidic form (HA) will predominate. If the pH is greater than the pKa, the basic form (A^-) will predominate. This comparison allows you to determine whether the acid will primarily exist in its protonated (acidic) or deprotonated (basic) form, which is crucial for understanding the acid's behavior in different pH environments.